Add "toll-free bridging" between Objective-C and CoreFoundation types

[madsmtm]

Certain CF types are toll-free bridged to Objective-C types.

We should support that, it'll probably be enough to just add AsRef implementations, as we have a blanked impl<T: AsRef<U>> From<&T> for Retained<U>.

This won't be perfectly efficient, it will incur an extra retain/release, but a direct conversion like From<CFRetained<CFString>> for Retained<NSString> probably won't be doable due to the orphan rule.

I guess we could add something like CFRetained::into_objc / CFRetained::from_objc too?